The present invention relates to a dielectric ceramic composition suitable for a nickel electrode multilayer ceramic capacitor in which low temperature sintering can be carried out, and in particular, relates to a dielectric ceramic composition and to a multilayer ceramic capacitor using the same, which satisfy X7R characteristics as a temperature characteristic of capacitance and which have reduction resistance in which there is little dielectric loss at or above room temperature.
Hitherto, compositions consisting of barium titanate are widely used as dielectric materials for multilayer ceramic capacitors, since the dielectric constant is high, the temperature characteristic is superior, the bias dependence is small, and the withstand voltage is superior.
Generally, a multilayer ceramic capacitor is produced by the following method. That is, dielectric powder such as barium titanate is mixed with an organic binder; then, the mixture is suspended and is formed in the shape of a sheet by the doctor blade method, and a dielectric ceramic layer is thereby produced. Furthermore, metal powder for an internal electrode is mixed with organic compounds such as organic solvent, plasticizer, and organic binder, to form a metal powder paste, and the paste is printed on the above dielectric ceramic layer by screen printing. Next, the dielectric ceramic layers are dried and are then alternatingly laminated and adhered; organic components are then removed by a heating treatment at about 300xc2x0 C. in air and are sintered at 1000xc2x0 C. or more. Subsequently, external electrodes are bonded by heating at both ends of the dielectric ceramic layer, and a multilayer ceramic capacitor is thereby produced. Although the sintering temperature is generally about 1000xc2x0 C. when lead is used in the dielectric ceramic composition, it is necessary to carry out sintering at about 1300xc2x0 C. or more when barium titanate is used therein because sintering characteristics are different. Thus, in the production method for a multilayer ceramic capacitor, the dielectric layers and the internal electrode metal layers are sintered at the same time.
As metals used in the internal electrode of the multilayer ceramic capacitor, heretofore, noble metals such as silver, palladium, platinum, gold, etc., in powder form, or base metals such as nickel, cobalt, iron, molybdenum, tungsten, etc., in powder form have been used, but since less expensive electronic materials are recently required, development of a multilayer ceramic capacitor using the latter base metal powder, in particular, metallic nickel powder, is widely employed. A sintering process in the above production method for the multilayer ceramic capacitor can also be carried out in an oxygen atmosphere when a noble metal powder is used in the internal electrode. However, when a base metal such as nickel is used therein, an insulation oxide is formed in an oxidizing atmosphere at high temperatures. Therefore, it is necessary to carry out the sintering in a reducing atmosphere such as hydrogen gas in this case.
In the production method for the multilayer ceramic capacitor, in order to remove organic constituents, an oxidation-reduction reaction is generated by carrying out heat treatment in air, or by carrying out the sintering in a reducing atmosphere in the case of the nickel electrode, as described above, and a volume change due to expansion and shrinkage is generated in the metal powder. In addition, a volume change is also generated in the dielectric by sintering; however, sintering characteristics such as volume changes due to expansion and shrinkage of each material in sintering, etc., are different, since different materials such as the dielectric and the metal powder are sintered at the same time. Warp is thereby generated in a metal paste layer, and as a result, there is a problem in that structural defects in which the layered structure is destroyed by a so-called xe2x80x9cdelaminationxe2x80x9d such as crack or peeling. Therefore, as a dielectric material, materials which can be sintered at as low a temperature as possible are required.
In addition, as a dielectric material, heretofore, materials which maintain an equilibrium state with oxygen in air were used, since they form oxides. However, in dielectrics used therefor, reduction resistance (or non-reducibility) is required, in which the characteristics are not affected by oxygen partial pressure, that is, in which it is reduced, does not turn into semiconductor, and is stable, even if the oxygen partial pressure is low, since the multilayer ceramic capacitor in the nickel electrode is sintered in the reducing atmosphere described above.
Therefore, in order to solve the above problem, various research has been conducted with regard to non-reducing dielectric ceramic compositions. Specifically, a multilayer ceramic chip capacitor comprising a dielectric layer containing BaTiO3 as a main component, and containing MgO, Y2O3, at least one of BaO and CaO, SiO2, MnO, and at least one of V2O5, and MoO3, as a subcomponent, wherein the subcomponent comprises 0.1 to 3 mol of MgO, 5 mol or less of Y2O3, 2 to 12 mol of BaO and CaO, 2 to 12 mol of SiO2, 0.5 or less mol of MnO, 0 to 0.3 mol of V2O5, 0 to 0.3 mol of MoO3, and more than 0 mol of V2O5 and MoO3, when BaTiO3 is 100 mol, is disclosed in Japanese Unexamined Patent Publication No. 8-124785.
In addition, a dielectric ceramic composition comprising a main component in which x and y satisfy 0.01xe2x89xa6xxe2x89xa60.1 and 0.1 less than y less than 0.26, respectively, when the chemical formula of the main component is represented by (Ba1xe2x88x92xCax)(Ti1xe2x88x92yZry)O3, and comprising 0.2 to 1 weight parts of Y2O3, 0.06 to 0.6 weight parts of MnO2, 0.1 to 1 weight parts of Al2O3, 0.1 to 1 weight parts of NiO and 0.1 to 1 weight parts of (1xe2x88x92xcex1)Li2Oxe2x88x92xcex1SiO2(0.3 less than xcex1(mole ratio) less than 0.7), to 100 parts of the main component, is disclosed in Japanese Unexamined Patent Publication No. 10-139538.
Furthermore, a dielectric ceramic composition comprising a barium titanate solid solution and additional components and comprising a sintering promoter as a subcomponent, wherein a main component is within an range of 0.950xe2x89xa6A/B (mole ratio)xe2x89xa61.050, 0.12 less than axe2x89xa60.30, and 0.04xe2x89xa6bxe2x89xa60.30, when the chemical formula of the dielectric ceramic composition is represented by ABO3 +aR+bM (ABO3 is a barium titanate solid solution; R is an oxide of metal elements selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Th, Dy, Ho, Er, Tm, Yb, and Lu; M is oxide of metal elements selected from Ni, Mg, Fe, Al, Cr, and Zn; and a and b are the mole ratio converted into the chemical formula including one metal element of each oxide).
The above conventional technologies improve reduction resistance and low temperature sintering of the dielectric material, and in addition, satisfy X7R characteristics, and they have moderate effects in reliability, bias characteristics, etc. However, with recent demands such as for rapid miniaturization of portable telephones, personal computers, etc., or increase in capacitance, a dielectric layer of the multilayer ceramic capacitor is required to be thin, and the thickness thereof is required at present to be 10 xcexcm or less. In this case, there is a problem in that the withstand voltage between a pair of internal electrodes is reduced when the dielectric layer is thinned. In addition, since it is required to have low electric power consumption form the viewpoint of energy conservation, dielectric materials which have low dielectric loss at or above room temperature and are exothermic, are desired.
It is an object of the present invention to provide a dielectric ceramic composition suitable for a multilayer ceramic capacitor in which an internal electrode is made of nickel, and to provide a multilayer ceramic capacitor using the same. Specifically, it is an object thereof to provide a dielectric ceramic composition in which the dielectric constant is high; low temperature sintering can be carried out in a reducing atmosphere; X7R characteristics are satisfied; and reduction resistance is such that there is little dielectric loss at or above room temperature, and to provide a multilayer ceramic capacitor having superior bias characteristics.
The inventors have conducted various research with regard to the above problems which were unsolved in the prior art, and they have researched with regard to type and compounding ratio of rare earth elements, using barium titanate as a main component. As a result, the inventors have found that a dielectric ceramic composition satisfies X7R characteristics and has little dielectric loss at or above room temperature and that a multilayer ceramic capacitor using the dielectric ceramic composition is superior in bias characteristics, and the present invention was thereby completed.
That is, a dielectric ceramic composition according to a first embodiment of the present invention has been made on the basis of the above knowledge, and it is characterized by comprising barium titanate, as a main component and comprising magnesium oxide, dysprosium oxide, barium oxide, calcium oxide, and vanadium oxide, as a subcomponent, wherein magnesium oxide converted into MgO is 1 to 3 mol, dysprosium oxide converted into Dy2O3 is 1 to 5 mol, the total of barium oxide and calcium oxide converted into BaO and CaO, respectively, is 0.1 to 5 mol, and vanadium oxide converted into V2O5 is 0.01 to 0.1 mol, when barium titanate converted into BaTiO3 is 100 mol.
A dielectric ceramic composition according to a second embodiment of the present invention is characterized by comprising barium titanate, as a main component and comprising magnesium oxide, dysprosium oxide, barium oxide, calcium oxide, silicon oxide, manganese oxide, and at least one of tantalum oxide, tungsten oxide, and vanadium oxide, as a subcomponent, wherein magnesium oxide converted into MgO is 1 to 3 mol, dysprosium oxide converted into Dy2O3 is 1 to 5 mol, the total of barium oxide and calcium oxide converted into BaO and CaO, respectively, is 0.1 to 5 mol, silicon oxide converted into SiO2 is 1 to 5 mol, manganese oxide converted into MnO is 0.1 to 1 mol, and the total of tantalum oxide, tungsten oxide, and vanadium oxide converted into Ta2O5, WO3, and V2O5, respectively, is 0.01 mol or more and 1 mol or less, when barium titanate converted into BaTiO3 is 100 mol. In the second embodiment, it is preferable that vanadium oxide be comprised as an essential component, and that at least one of tantalum oxide and tungsten oxide be comprised.
In addition, a dielectric ceramic composition according to a third embodiment of the present invention is characterized by comprising barium titanate, as a main component, and comprising magnesium oxide, dysprosium oxide, barium oxide, calcium oxide, silicon oxide, manganese oxide, and at least one of tantalum oxide, tungsten oxide, vanadium oxide, and molybdenite oxide, as a subcomponent, wherein magnesium oxide converted into MgO is 1 to 3 mol, dysprosium oxide converted into Dy2O3 is 1 to 5 mol, the total of barium oxide and calcium oxide converted into BaO and CaO, respectively, is 0.1 to 5 mol, silicon oxide converted into SiO2 is 1 to 5 mol, manganese oxide converted into MnO is 0.1 to 1 mol, and the total of tantalum oxide, tungsten oxide, vanadium oxide, and molybdenite oxide converted into Ta2O5, WO3, V2O5, and MoO3, respectively, is 0.01 mol or more and 1 mol or less, when barium titanate converted into BaTiO3 is 100 mol. In the third embodiment, it is preferable that vanadium oxide be comprised as an essential component, and that at least one of tantalum oxide, tungsten oxide, and molybdenite oxide be comprised.
Furthermore, a multilayer ceramic capacitor according to the present invention is characterized by using the above dielectric ceramic composition. In the following explanation, the mole amount of oxide refers to an amount at which barium titanate converted into BaTiO3 is 100 mol.
According to the first dielectric ceramic composition of the present invention, sintering characteristics are improved by comprising dysprosium oxide, barium oxide, calcium oxide, and vanadium oxide in a dielectric ceramic composition in which barium titanate is a main component, in particular, by comprising dysprosium oxide, as a rare earth element, and therefore, a dielectric ceramic composition, which satisfies X7R characteristics even in sintering at a low temperature of 1300xc2x0 C. or less, and which has hardly any dielectric loss at or above room temperature, can be obtained.
In particular, the dielectric ceramic composition has a dielectric loss in a high temperature region of 30 to 150xc2x0 C. which is 0.5% or less and which is much smaller than those of conventional compositions, and it is superior to reliability. Therefore, the dielectric ceramic composition of the present invention is useful as a dielectric material or as an intermediate material for a high dielectric constant ceramic capacitor, an axial condenser, a disk condenser, thick film capacitor, etc, as well as for a nickel electrode multilayer ceramic capacitor.
In addition, since dielectric characteristics are superior even in sintering at 1300xc2x0 C. or less, the rate of occurrence of structural defects such as delamination decreases in the production of the multilayer ceramic capacitor.
Furthermore, the effect of the present invention is further improved by adding the following components to the above components.
1) silicon oxide
2) silicon oxide, and manganese oxide
3) silicon oxide, manganese oxide, and molybdenum oxide
4) tantalum oxide, silicon oxide, and manganese oxide
5) tungsten oxide, silicon oxide, and manganese oxide
6) silicon oxide, manganese oxide, and at least one of tantalum oxide and tungsten oxide
The contents of the above components is 1 to 5 mol of silicon oxide converted into SiO2, 0.1 to 1 mol of manganese oxide converted into MnO, 0.05 to 0.2 mol of molybdenum oxide converted into MoO3, 0.01 to 1 mol of tantalum oxide converted into Ta2O5, and 0.01 to 1 mol of tungsten oxide converted into WO3, and the total of tantalum oxide and tungsten oxide converted into Ta2O5 and WO3, respectively, is 0.01 mol or more and 1 mol or less.
The multilayer ceramic capacitor using the above dielectric ceramic composition is superior to those used in the conventional dielectric ceramic compositions in bias characteristics in a high voltage region as well as static capacitance and temperature characteristics.
In the following, the reasons for the above numerical limitations will be explained with the effects of the present invention.
Magnesium Oxide: 1 to 3 mol Converted into MgO
When the content of magnesium oxide is under 1 mol, the temperature changing rate of capacitance deteriorates, and it is difficult to decrease the aging deterioration of capacitance. In contrast, when the content of magnesium oxide exceeds 3 mol, the sintering characteristics rapidly deteriorate, the density thereof is insufficient, and thereby the insulation resistance accelerated life is reduced and a high dielectric constant is not obtained. Therefore, the content of magnesium oxide is specified to be 1 to 3 mol converted into MgO. The more preferable content thereof is 1 to 2 mol.
Dysprosium Oxide: 1 to 5 mol Converted into Dy2O3 
Dysprosium oxide adjusts the dielectric constant and the insulation resistance of the dielectric porcelain composition, and it has an effect in which the insulation resistance accelerated life is improved and is thereby also improved in the DC bias characteristics. In addition, the largest effect of dysprosium oxide is to remarkably improve the sintering characteristic in comparison with rare earth metals such as yttrium oxide, and the sintering at low temperatures can thereby be carried out. When the content of dysprosium oxide is under 1 mol, the above effect due to addition becomes insufficient, and in particular, the DC bias characteristics become insufficient. In contrast, when the content of dysprosium oxide exceeds 5 mol, the dielectric constant is reduced. Therefore, the content of dysprosium oxide is specified to be 1 to 5 mol converted into Dy2O3. The more preferable content thereof is 1 to 2 mol.
Barium Oxide and Calcium Oxide: Total 0.1 to 5 mol Converted into BaO and CaO
When the total content of barium oxide and calcium oxide is under 0.1 mol, the aging deterioration of capacitance in loading DC electric field increases. In addition, the insulation resistance accelerated life becomes insufficient, and the temperature characteristics of capacitance deviate from the desire range. In contrast, when the total content of barium oxide and calcium oxide exceeds 5 mol, the insulation resistance accelerated life becomes insufficient and the dielectric constant rapidly decreases. Therefore, the total content of barium oxide and calcium oxide is specified to be 0.1 to 5 mol converted into BaO and CaO, respectively. The more preferable total content thereof is 0.3 to 3 mol, and the most preferable total content thereof is 0.5 to 1.5 mol. The content of barium oxide is preferably 0.1 to 3 mol, and more preferably 0.3 to 1.5 mol, converted into BaO. The content of calcium oxide is preferably 0.1 to 3 mol, and more preferably 0.3 to 1.5 mol, converted into CaO.
Vanadium Oxide: 0.01 to 0.1 mol Converted into V2O5 
The vanadium oxide improves not only aging deterioration of capacitance under DC electric field, but also the dielectric breakdown voltage. When the content of the vanadium oxide is under 0.01 mol, it is difficult to obtain the effects. In contrast, when it exceeds 0.1 mol, extreme reduction of the initial insulation resistance is caused. Therefore, the content of vanadium oxide is specified to be 0.01 to 0.1 mol converted into V2O5. The content is preferably 0.02 to 0.1 mol.
In the present invention, the present invention is further improved by adding the following oxides.
Silicon Oxide: 1 to 5 mol Converted into SiO2 
When the content of silicon oxide is under 1 mol, the sinterability is reduced, and thereby the density thereof is easily insufficient. In contrast, when the content exceeds 5 mol, the initial insulation resistance tends to be too low. Therefore, it is desirable that the content of silicon oxide converted into SiO2 be 1 to 5 mol. The content is preferably 1 to 3 mol.
Manganese Oxide: 0.1 to 1 mol Converted into MnO
Manganese oxide increases the density of the dielectric layer and improves dielectric loss tan xcex4 of the dielectric ceramic composition, and in addition, it has an effect in which the insulation resistance accelerated life is improved. When the content of manganese oxide is under 0.1 mol, it is difficult to obtain the effect. In contrast, when the content of manganese oxide exceeds 1 mol, it is difficult to decrease the aging deterioration of capacitance in loading a DC electric field. Therefore, it is desirable that the content of manganese oxide be 0.1 to 1 mol converted into MnO. The content is preferably 0.05 to 0.15 mol.
Molybdenum Oxide: 0.05 to 0.2 mol Converted into MoO3 
Molybdenum oxide improves the aging deterioration of capacitance under a DC electric field, and in addition, the accelerated life of insulation resistance. When the content of molybdenum oxide is under 0.05 mol, it is difficult to obtain the effect. In contrast, when it exceeds 0.2 mol, extreme reduction of the insulation resistance easily occurs. Therefore, it is desirable that the content of molybdenum oxide be 0.05 to 0.2 mol converted into MoO3. The content is preferably 0.05 to 0.1 mol.
Tantalum Oxide: 0.01 to 1 mol Converted into Ta2O5 
Tantalum oxide improves the aging deterioration of capacitance in the same manner as vanadium oxide. Furthermore, it has also an effect of slightly increasing the dielectric constant. When the content of tantalum oxide is under 0.01 mol, it is difficult to obtain the effect. In contrast, when it exceeds 1 mol, reduction of the initial insulation resistance occurs. Therefore, the content of tantalum oxide is specified to be 0.01 to 1 mol converted into Ta2O5.
Tungsten Oxide: 0.01 to 1 mol Converted into WO3 
Tungsten oxide improves the aging deterioration of capacitance in the same manner as vanadium oxide and tantalum oxide. Furthermore, it has also an effect of slightly increasing the dielectric constant. When the content of tungsten oxide is under 0.01 mol, it is difficult to obtain the effect. In contrast, when it exceeds 1 mol, reduction of the initial insulation resistance occurs. Therefore, the content of tungsten oxide is specified to be 0.01 to 1 mol converted into WO3.
In the present invention, at least one of the above tantalum oxide and tungsten oxide may be contained, but when both are contained, it is desirable that they be contained within 0.05 molxe2x89xa6Ta2O5+WO3xe2x89xa61 mol.
In the second dielectric ceramic composition of the present invention, it is necessary that the total amount of tantalum oxide, tungsten oxide, and vanadium oxide satisfy 0.01 molxe2x89xa6Ta2O5+WO3+V2O5xe2x89xa61 mol converted into Ta2O5, WO3, and V2O5, respectively. Vanadium oxide also has the same functions as those of tantalum oxide and tungsten oxide. By containing total 0.01 mol or more of these oxides, the dielectric breakdown voltage is improved, as well as the aging deterioration of capacitance under a DC electric field. The extreme reduction of the initial insulation resistance occurs when the total amount of the above oxides exceeds 1 mol.
In the third dielectric ceramic composition of the present invention, it is necessary that the total amount of tantalum oxide, tungsten oxide, vanadium oxide, and molybdenum oxide satisfy 0.01 molxe2x89xa6Ta2O5+WO3+V2O5+MoO3xe2x89xa61 mol converted into Ta2O5, WO3, V2O5, and MoO3, respectively. Molybdenum oxide also has the same functions as those of tantalum oxide to vanadium oxide. By containing total 0.01 mol or more of these oxides, the dielectric breakdown voltage is improved, as is the aging deterioration of capacitance under a DC electric field. The extreme reduction of the initial insulation resistance occurs when the total amount of the above oxides exceeds 1 mol.
In addition, it is desirable that the average crystalline particle diameter of the dielectric ceramic composition in the present invention be 2 xcexcm or less. Thus, a thin dielectric layer can be formed, and therefore, the multilayer ceramic capacitor can be stably produced. In order to obtain the average crystalline particle diameter, it is desirable that the average particle size of raw material powder be 0.5 to 1.5 xcexcm.
The property of barium titanate used in the present invention is not specifically limited, but in particular, it is preferable that the average particle size thereof be 0.1 to 1 xcexcm and that the specific surface thereof be 1 to 10. As a production method for barium titanate used as a raw material, well-known methods such as a solid phase method, hydrothermal method, etc., can be used. However, methods using liquid-phase methods (called a xe2x80x9csol-gel methodxe2x80x9d or a xe2x80x9csynthesis colloid methodxe2x80x9d, etc.) are more preferable than the above methods using reactions by heating to a high temperature. Specifically, as the methods, an aqueous solution of a titanium compound such as titanium tetrachloride, etc., and an aqueous solution of barium compound such as barium hydroxide, barium chloride, etc., are reacted by contacting each other in an alkali range at 100xc2x0 C. or less. More specifically, an aqueous solution of a titanium compound and an alkali aqueous solution of barium compound are contacted with each other while stirring and controlling the mole ratio of titanium compound and barium compound to 0.8 to 1.2.
As a titanium compound, at least one selected from halide, hydroxide, nitrate, sulfate, acetate, perchlorate, oxalate, and alkoxide, can be used. As a specific titanium compound, titanium tetrachloride, titanium trichloride, titanium hydroxide, titanyl sulfate, etc., can be mentioned, and of these, titanium tetrachloride can be preferably used. As a barium compound, at least one selected from halide, hydroxide, nitrate, sulfate, acetate, perchlorate, oxalate, and alkoxide, can be used. As a specific barium compound, barium chloride, barium hydroxide, barium nitrate, barium sulfate, barium acetate, etc., can be mentioned, and of these, barium chloride or barium hydroxide can be preferably used. In addition, barium hydroxide, which is produced by preliminarily contacting a hydroxide of an alkali metal such as NaOH, KOH, etc., with barium salt compound (halide, nitrate, sulfate, acetate, etc.) such as barium chloride, and by reacting by heating, may be used. Furthermore, the barium titanate obtained as above may be used after pre-sintering.
In the production method of the dielectric ceramic composition of the present invention, each component described as above is mixed in a specific amount and is then subjected to a dry-type or wet-type classification and blending. In order to more uniformly disperse and mix, the latter wet-type blending is preferable, and it is also preferable to add dispersing agent such as ammonium polyacrylate in this case. Subsequently, the mixture of barium titanate and subcomponents is sintered by heating. Each subcomponent can be used in the form of oxide as a raw material, and in particular, it can also be used in the form of carbonate and hydroxide as a raw material in order to improve the other reactivities.
In the case in which silicon oxide is comprised as the other component in the dielectric ceramic composition of the present invention, the dielectric ceramic composition may be produced by preliminarily mixing silicon oxide alone. However, it is desirable that the dielectric ceramic composition be produced by mixing 3 components of silicon compounds such as silicon oxide, etc.; barium compounds such as barium oxide, barium carbonate, etc.; and calcium compounds such as calcium oxide, calcium carbonate, etc., and by pre-sintering, and then, by sintering after mixing with other components. In this case, the temperature of the pre-sintering is preferably 800 to 1100xc2x0 C., and is more preferably 1000xc2x0 C. The characteristics of the dielectric ceramic composition, in particular, the dielectric life at which the multilayer ceramic capacitor is constituted, is improved by using this production method.
Next, an example of a production method for a multilayer ceramic capacitor in which a dielectric ceramic composition of the present invention and in which an internal electrode is made of nickel is used will be explained in the following.
Firstly, an organic vehicle in which a powder of the dielectric ceramic composition of the present invention and a binder such as ethyl cellulose, etc., are dissolved in an organic solvent is kneaded, and a dielectric paste is prepared. On the other hand, metallic nickel super fine powder having an average particle size of 0.2 to 0.4 xcexcm is kneaded with the above organic vehicle, and a paste for a metal electrode is prepared. Next, the dielectric paste is formed in a sheet by a doctor blade method, and a dielectric greensheet is produced. On the other hand, the metallic nickel paste for an internal electrode is printed on the above greensheet by a screen printing. They are alternately laminated, and a green chip is formed by cutting in the desire shape. Subsequently, the green chip is heated at 250 to 300xc2x0 C. in air, and a binder removing process is carried out. Next, it is sintered within a temperature range of 1100 to 1300xc2x0 C. in a reducing atmosphere such as hydrogen gas, and a multilayer ceramic capacitor is formed. Then, it is annealed in an oxygen atmosphere in a temperature range of 500 to 1000xc2x0 C., and a process for reoxidizing the dielectric layer is carried out.